The invention relates to the field of wavelength division multiplex (WDM) optical transmission systems, and more particularly to optical fiber equipment having a Bragg grating tunable by a piezoelectric actuator.
The invention is based on a priority application EP 01 440 235.8 which is hereby incorporated by reference.
Attempts are being made to increase the capacity of WDM type systems, and indeed of dense WDM (DWDM) type systems, by increasing the data rate of each channel and/or by increasing the number of channels in the transmission windows, while nevertheless avoiding detection errors as caused for example by the data transmitted in the channels being deformed because of chromatic dispersion or temperature drift of the transmission line when subjected to a varying climatic environment.
Thus, in the prior art, wavelength-selective optical filters such as Bragg gratings are used, for example, for optical routing purposes in order to add or extract wavelengths by means of an optical add-drop multiplexer (OADM) or an optical cross-connect (OXC) unit.
In addition, wavelength-tunable Bragg gratings make it possible to select one channel or another channel, for example, as a function of the wavelength to be passed, or to tune onto a channel or between two channels so as to allow the system to pass light or prevent it from passing light. Bragg gratings are also used for compensating the chromatic dispersion of the various channels of a transmission line. Bragg gratings are temperature compensated if they are exposed to a varying climatic environment.
A Bragg grating is photoinduced in the core and/or in one or more layers of cladding in an optical fiber, and it comprises substantially periodic modulation of the refractive index in the core and/or in one or more layers of cladding of the fiber in the presence of photosensitive dopants. The pitch of the grating, i.e. its modulation period along the fiber, determines the spectral response of the grating which is centered around a wavelength known as the xe2x80x9cBragg wavelengthxe2x80x9d. With a piezoelectric actuator, it is possible to modify the pitch along the fiber and thus to modify the Bragg wavelength so as to counter the problems mentioned above. An actuator comprises a stack of piezoelectric layers individually connected to pairs of electrodes and electrically insulated from one another, the layers being stuck together.
European patent application EP 0 855 608 discloses an embodiment of optical fiber equipment having a Bragg grating tunable by a piezoelectric actuator. In that equipment, the piezoelectric actuator is fixed mechanically via the ends of its stack to an optical fiber support having two uprights, each having a fiber-holding element. When a voltage is applied to the actuator, it lengthens longitudinally under the effect of axial stress which is transferred to the Bragg grating via the two uprights of the support. As a result, the Bragg grating is subjected to the same elongation as the actuator.
In that configuration, the piezoelectric actuator is not restrained sufficiently at its ends. When a voltage pulse is applied, the actuator is subjected temporarily to an overshoot stage: it begins by lengthening beyond the elongation appropriate for the applied voltage and it then shrinks and reaches the appropriate elongation only after a period during which it oscillates. This excess elongation generates vibration and mechanical stresses that damage the layers of the actuator which can suffer irreversible microfractures within the stack of layers, thereby permanently degrading the response of the piezoelectric actuator. In addition, these stresses are communicated to the Bragg grating, thus spoiling its spectral response. In addition, these instabilities increase the switching time of the equipment, i.e. the time necessary for adjusting the spectral response of the Bragg grating as a function of events.
An object of the present invention is to implement optical fiber equipment having a Bragg grating tunable by a piezoelectric actuator that has switching time that is shorter than in the prior art, that is reliable, and that is strong, thereby increasing its lifetime, particularly by greatly reducing overshoot. The cost of the equipment is also low, it is simple to manufacture, and its dimensions are as small as possible.
To this end, the present invention provides optical fiber equipment having a Bragg grating tunable by a piezoelectric actuator, the equipment comprising an optical fiber, a Bragg grating photoinduced in said fiber, a support for said fiber, the support having two uprights at opposite ends of said Bragg grating and each upright having a first holding element for holding said fiber, and a piezoelectric actuator held via its ends between the two uprights, the equipment being characterized in that said actuator is prestressed longitudinally in compression by said uprights.
The equipment of the invention thus has an actuator mounted in a prestressed condition on initial assembly by means of the uprights of the support. This prestress is applied and adjusted by selecting a distance between the uprights at the actuator which is shorter than the length of the actuator. With the actuator compressed in this way, the excess elongation of the actuator during an overshoot stage is practically zero.
In addition, practically eliminating overshoot duration has the effect of reducing switching time. Furthermore, the reduction in switching time provided by the invention avoids activating the resonant modes of the support which are sources of vibration and which consequently interfere with the spectral response of the Bragg grating.
The compression created needs to be adjusted depending on the type of piezoelectric actuator. The initial compression must not prevent the actuator from lengthening when subjected to a voltage. With increasing prestress force, the extent to which the piezoelectric actuator lengthens decreases, and consequently the tuning range decreases. If the prestress force decreases, then the extent to which the piezoelectric actuator can lengthen increases and consequently the tuning range increases. A compromise must be found between a suitable tuning range and minimizing switching time.
After maximum elongation corresponding to maximum operating voltage, the compression state of the actuator must not be too great. The pre-loaded support is preferably made from a material of stiffness and elastic limit such that the elongation of the piezoelectric actuator is properly transferred to the fiber. The thermal expansion coefficient of this material must also be small so that variation in the temperature of the assembly has little influence on the initial prestress state, and so that temperature drift of the assembly is minimized.
Furthermore, the equipment of the invention makes it possible to achieve a phenomenon whereby the deformation of the optical fiber is amplified, thereby increasing the tunability of the Bragg grating of the invention.
Advantageously, the prestress that is created can be substantially less than or equal to half the null force of the actuator.
The null force of the actuator is defined as follows. When the maximum operating voltage is applied to an actuator that is not subjected to an external force, then maximum elongation is obtained. This elongation decreases with increasing external force exerted on the ends of the actuator starting from said state. The null force is defined as the force above which elongation of the actuator becomes zero.
In the invention, the actuator is subjected to compression via the uprights of the support before any voltage is applied. To minimize mechanical damage to the actuator, the amount of prestress that is created is advantageously selected to be less than or equal to half the null force of the actuator.
The support of the invention can be made of a material based on iron and on nickel.
This type of material presents mechanical properties that are suitable for the invention. Stiffness is sufficient but not too large, so it is possible for the support to deform, while elastic limit is high so that deformation remains within the reversible range. In addition, the coefficient of thermal expansion can be selected to be small in the temperature range extending from xe2x88x92100xc2x0 C. to +200xc2x0 C.
In a preferred embodiment, the support of the invention includes a cross-member disposed perpendicularly between the two uprights and holding them together.
The support comprising the two uprights and the cross-member is constituted by a single piece. The cross-member reinforces the structure of the support with stiffness that is infinite at its center.
In this embodiment, the support can have a plurality of recesses disposed at the intersections between the cross-member and the two uprights.
Such recesses reduce the stiffness of the support, thereby allowing it better flexibility. The recesses can thus be used to adjust the value of the prestress so as to allow the actuator to move over its entire operating range. When a voltage is applied, the actuator lengthens and the uprights move apart and pivot about centers of rotation disposed within the recesses. Adding such recesses thus reinforces the phenomenon whereby displacement of the optical fiber is amplified. Depending on the position of the cross-member, this displacement of the fiber can be in the same direction or in the opposite direction to the deformation of the actuator.
In one configuration of this embodiment, the cross-member is placed at the ends of the two uprights opposite from their ends carrying the first holding elements, the support being U-shaped and the actuator being disposed above the cross-member.
The cross-member then serves to stabilize the support.
In this configuration, the cross-member can have a base including a portion of extra thickness substantially centered in its middle. This portion of extra thickness serves firstly to provide better pivoting of the two uprights during displacement of the piezoelectric actuator, and secondly to fix the assembly securely to its mounting plane.
In addition, in this configuration, elongation of the actuator gives rise to elongation of the optical fiber containing the Bragg grating. Thus, by means of this elongation, the Bragg wavelength of the grating increases.
In another configuration of this embodiment, the cross-member is disposed between the actuator and the optical fiber, so that the support is H-shaped.
In addition, in this other configuration, when the piezoelectric actuator lengthens, the optical fiber containing the Bragg grating shortens. Thus, when the piezoelectric actuator lengthens, the Bragg wavelength of the grating decreases.
Furthermore, the height between the horizontal axis of the cross-member and the horizontal axis of the optical fiber, and the height between the horizontal axis of the cross-member and the height of the actuator can both be selected as a function of the characteristics of the piezoelectric actuator and of the desired tuning range.
In this other configuration, the equipment of the invention can be insensitive to temperature.
This is achieved by suitably selecting the coefficients of thermal expansion of the materials of the actuator and of the support. In this manner, any temperature fluctuation is automatically compensated by the cross-member deforming appropriately.
In addition, each upright of the invention can have a second fiber-holding element disposed adjacent to its first holding element on its side remote from the Bragg grating.
An optical fiber comprises a core and one or more layers of cladding all made of a rigid material of silica type, and surrounded by a more flexible outer coating made of a material of polymer type. At the first holding elements and also between them, the optical fiber of the invention need not have an outer coating so that the fiber can be held firmly in order to transmit thereto all of the deformation imparted by the support during displacement of the piezoelectric actuator, and consequently impart elongation to the Bragg grating. These first holding elements are fixed either by adhesive or by soldering directly to the cladding of the fiber in order to be able to communicate all of the deformation of the actuator to the optical fiber.
The second holding elements on the outsides of the two uprights strengthen the output leads optical fiber in order to prevent it being torn away. These two holding elements are fixed by adhesive to the coating of the fiber and they are spaced far enough apart from the first holding elements to avoid the chemical components of the fixing means diffusing into one another.
Advantageously, the equipment of the invention can have a plurality of strain gages associated with a servo-control loop for controlling deformation of the support.
The strain gages, e.g. three strain gages arranged in a star or a delta configuration, serve to measure deformation of the support and thus monitor lengthening of the actuator. By associating them in a servo-control loop providing feedback to the electrical feed to the piezoelectric actuator, these strain gages serve to correct for hysteresis in the response of the actuator to an applied voltage, and thus to obtain elongation that depends linearly on the applied voltage. In addition, this association can correct for possible temperature drift of the Bragg grating.